7.1.2 IEC listening rooms
The first type of critical listening room is the IEC listening room (IEC, 2003). This is essentially a conventional room that meets certain minimum requirements: a reverberation time that is flat, and between 0.3 and 0.6 seconds above 200 Hz, a low noise level, an even mode distribution and a recommended floor area.
In essence this is a standardized living room that provides a consistent reference environment for a variety of listening tasks. It is the type of room that is often used for psychoacoustic testing as it provides results that correlate well with that which is experienced in conventional domestic environments. This type of room can be readily designed using the techniques discussed in Chapter 6.
However, for critically listening to music mixes, etc. something more is required and these types of room will now be discussed. All of them don't only control reverberation, but also the time evolution and level of early reflections. They also all take advantage of the fact that the speakers are in specific locations to do this and very often have an asymmetric acoustic that is different for the listener and the loudspeakers.
Although there are many different implementations, they fall into three basic types: reflection controlled rooms, non-environment rooms, and diffuse reflection rooms. As they all control the early reflections within a room we shall look at them first.
7.1.3 Energy"time considerations
A major advance in acoustical design for listening to music has arisen from the realization that, as well as reverberation time, the time evolution of the first part of the sound energy build-up in the room matters, that is, the detailed structure and level of the early reflections, as discussed in Chapter 6. As it is mostly the energy in the sound that is important as regards perception, the detailed evolution of the sound energy as a function of time in a room matters.
Also there are now acoustic measurement systems that can measure the energy"time curve of a room directly, thus allowing a designer to see what is happening within the room at different frequencies, rather than relying on a pair of "golden ears." An idealized energy"time curve for a typical room is shown in Figure 7.3.
FIGURE 7.3 An idealized energy"time curve.
It has three major features:
- A gap between the direct sound and first reflections. This happens naturally in most spaces and gives a cue as to the size of the space. The gap should not be too long — less than 30 ms — or the early reflections will be perceived as echoes. Some delay, however, is desirable as it gives some space for the direct sound and so improves the clarity of the sound, but a shorter gap does add "intimacy" to the space.
- The presence of high-level diffuse early reflections, which come to the listener predominantly from the side, that is, lateral early reflections. This adds spaciousness and is easier to achieve over the whole audience in a shoebox hall rather than a fan-shaped one. The first early reflections should ideally arrive at the listener within 20 ms of the direct sound. The frequency response of these early reflections should ideally be flat and this, in conjunction with the need for a high level of lateral reflections, implies that the side walls of a hall should be diffuse reflecting surfaces with minimal absorption.
- A smoothly decaying diffuse reverberant field which has no obvious defects, and no modal behavior, and whose time of decay is appropriate to the style of music being performed. This is hard to achieve in practice so a compromise is necessary in most cases. For performing acoustic music a gentle bass rise in the reverberant field is desirable to add "warmth" to the sound but in studios this is less desirable.